1. Title of the Invention
This invention relates to an image forming apparatus which forms images on image bearing members and transfers the images onto a recording material conveyed by a recording material carrying means. The present invention can be suitably applied, for example, to an electrophotographic image forming apparatus, and particularly to a color electrophotographic copier which forms images having different colors on a plurality of image bearing members, such as electrophotographic photosensitive members, and sequentially transfers the respective images to the same recording material, superposing the images.
2. Description of the Related Art
There are known various kinds of color image forming apparatuses wherein a plurality of image forming units are provided, toner images having different colors are formed in the respective image forming units, and the toner images are sequentially transferred and superimposed onto the same recording material. Mostly used among them are color copiers using multicolor electrophotography.
An example of the aforementioned color electrophotographic copiers will be briefly explained with reference to FIG. 5. First, second, third and fourth image forming units Pa, Pb, Pc and Pd are provided within the main body of a color electrophotographic copier. The image forming units Pa, Pb, Pc and Pd include respective dedicated image bearing members; electrophotographic photosensitive drums 1a, 1b, 1c and 1d in the present example.
Latent image forming units 2a, 2b, 2c and 2d, developing units 3a, 3b, 3c and 3d, and cleaning units 5a, 5b, 5c and 5d are disposed near respective outer circumferences of the photosensitive drums 1a, 1b, 1c and 1d.
A recording material conveying belt 8 is disposed beneath the image forming units Pa, Pb, Pc and Pd. Transfer units 4a, 4b, 4c and 4d, each provided with a charger for transfer, are disposed inside the enclosed loop of recording material conveying belt 8.
In this configuration, a latent image corresponding to a cyan-component color in the image of an original is formed on the photosensitive drum la of the first image forming unit Pa by the latent image forming unit 2a. The latent image is made visible image using a developer comprising cyan toner contained in the developing unit 3a. The cyan-toner image is transferred to a recording material 6 comprising a paper-like sheet conveyed from a recording material cassette 60 by the conveying belt 8 via registration rollers 13 in the transfer unit 4a.
As the cyan-toner image is transferred to the recording material 6 as described above, a latent image corresponding to a magenta-component color in the image of the original is formed on photosensitive drum 1b in the second image forming unit Pb. A magenta-toner image formed on photosensitive drum 1b by the developing unit 3b is transferred to a predetermined position on the recording material 6 when the recording material 6 on which the image transfer in the first image forming unit Pa has been completed is conveyed to the transfer unit 4b by the conveying belt 8.
Subsequently, a yellow-toner image and a black-toner image are formed in the third and fourth image forming units Pc and Pd, respectively, in the same manner as described above. The yellow-toner image and the black-toner image are superimposed to the predetermined position on the above-described recording material 6 carried by the conveying belt 8.
After the completion of the above-described image forming processes, the respective toner images on the recording material 6 are fused together and fixed on the recording material 6 by a fixing unit 7, and thus a full-color image is obtained. After the completion of the transfer, toner particles remaining on the photosensitive drums 1a, 1b, 1c and 1d are removed by the cleaning units 5a, 5b, 5c and 5d so that the photosensitive drums are ready for the next latent image formation.
When the conveying belt 8 starts to rotate, the recording material 6 is conveyed onto the belt 8 via the registration rollers 13. At that time, an image writing signal is turned on, and image formation is performed at a predetermined timing on the photosensitive drum 1a for the first color. When the recording material 6 is conveyed to the contact portion (the transfer position) of the photosensitive drum 1a and the conveying belt 8, the toner image on the photosensitive drum 1a is transferred onto the recording material 6. At that time, the electrostatic attraction of recording material 6 to the conveying belt 8 is high due to a voltage applied to the charger for transfer.
In some cases, however, the front end of recording material 6 is floating when it is fed onto the conveying belt 8. Hence, particularly when a recording material having strong stiffness, such as with cardboard, or having a curled recording material is used, the front end of the recording material may not adhere sufficiently to the conveying belt, causing insufficient transfer, the front end of the recording material may be bent, or a sufficient retention force on the conveying belt may not be obtained after transfer of the first-color image, causing paper jamming or paper folding when starting transfer of the second-color image.